Battery conservation in implantable cardioverter-defibrillators and pacemakers

ABSTRACT

In an implantable cardioverter-defibrillator and/or pacemaker, each having DDD pacing capabilities, an improved method of operation is described which dramatically increases the longevity of the implanted device by conserving battery power. The method comprises deactivating at least one unnecessary, power-consuming feature of the device until such feature is needed and then reactivating said feature only for so long as it is required by the patient. In a particular embodiment, the atrial sense amplifier is deactivated during normal operation of the implantable device, resulting in single-chamber sensing and pacing. Upon the occurrence of a predefined event, indicative of a need for dual-chamber sensing and pacing, the atrial sense amplifier is reactivated, the need for DDD pacing confirmed, and if appropriate, DDD pacing is begun. Once the patient&#39;s heart rate has returned to an acceptable level, the atrial sense amplifier is again deactivated and single-chamber sensing/pacing continued. In addition, the atrial sense amplifier of an ICD/pacemaker is deactivated during normal operation of the device and reactivated immediately following the detection of ventricular tachycardia. In this embodiment, DDD sensing/pacing is preferably automatically begun following this detection. Also contemplated herein, are improved devices employing the improved methods.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.09/418,925 filed Oct. 15, 1999 now U.S. Pat. No. 6,363,280.

FIELD OF THE INVENTION

The subject matter disclosed and claimed herein relates to an improvedmethod of operation of pacemakers and implantablecardioverter-defibrillators (ICDs) having pacing capabilities, whichimproved method serves to prolong battery longevity by deactivatingcertain power-consuming features while not needed and reactivating asnecessary. Also provided herein are improved devices employing thesemethods.

BACKGROUND OF THE INVENTION

The heart functions to pump life-sustaining blood throughout one's body.The human heart comprises a left side and a right side with each sidehaving a first chamber known as the atrium and a second chamber known asthe ventricle. The right atrium receives blood from the body after thebody has extracted the oxygen therefrom and the left atrium receivesoxygenated blood from the lungs. At an appropriate time, an electricalstimulus is provided to the atria that causes the muscle tissue todepolarize. Immediately following depolarization, the atrial muscletissue physically contracts, forcing the blood held in the right andleft atria through one-way valves into the right and left ventricles,respectively.

The electrical stimulus provided to the atria also stimulates theventricles after a delay which is sometimes referred to as the “naturalconduction time” of the heart. Upon stimulation, the ventricular muscletissue depolarizes and then contracts. This forces the blood held withinthe right ventricle to pass through the pulmonary artery to the lungsand the blood held within the left ventricle to pass through the aortato the rest of the body. In this manner, then, the heart “beats” orpumps blood by having the atria contract and, after the naturalconduction time, by having the ventricles contract. After a longerdelay, during which delay the right atrium is refilled with bloodreturning from throughout the body, the process repeats.

Implantable pacemakers and cardioverter-defibrillators (ICDs) areelectronic medical devices that monitor the electrical activity of theheart and provide electrical stimulation to one or more heart chambers,when necessary. For example, a pacemaker senses an arrhythmia, i.e., adisturbance in heart rhythm, and provides appropriate electricalstimulation pulses, at a controlled rate, to selected chambers of theheart in order to correct the arrhythmia and restore the proper heartrhythm. The type of arrhythmias that may be detected and corrected bypacemakers include bradycardias, which are unusually slow heart rates,and certain tachycardias, which are unusually fast heart rates.

Implantable cardioverter-defibrillators (ICDs) also detect arrhythmiasand provide appropriate electrical stimulation pulses to selectedchambers of the heart to correct the abnormal heart rate. In contrast topacemakers, however, the pulses from an ICD are much stronger and lessfrequent. This is because ICDs are generally designed to correctfibrillations, which are rapid, unsynchronized quiverings of one or moreheart chambers, and severe tachycardias, where the heart beats are veryfast but coordinated. To correct such arrhythmias, an ICD delivers alow, moderate or high energy shock to the heart. In addition tofunctioning as a cardioverter-defibrillator, some ICDs are designed toprovide pacing support to the heart. Such ICDs sense the occurrence of acardiac arrhythmia and automatically apply an appropriate therapy to theheart aimed at terminating the specific arrhythmia detected. This typeof therapy is referred to as “tiered therapy”.

In a tiered therapy ICD, each “tier” or level of therapy generallycorresponds to a different type of arrhythmia and typically to aspecified number of shocks of varying energies and pulse durationsintended to most efficiently terminate the specific type of arrhythmiadetected. Thus, such tiered therapy may include antitachycardia pacingfor painless termination of monomorphic ventricular tachycardia (i.e.,tachycardia that originates from one ventricular focus); programmablelow-energy cardioversion also for treatment of ventricular tachycardia(e.g., when antitachycardia pacing fails to terminate the tachycardia);high-energy defibrillation for termination of ventricular fibrillation;and back-up bradycardia pacing, for ensuring the heart beats,particularly following cardioversion or defibrillation. For examples oftiered therapy ICDs, see U.S. Pat. Nos. 4,427,011; 4,541,430; 4,398,536;and 5,103,822; each of which is incorporated herein, in its entirety, byreference.

Because the invention described and claimed herein is useful inpacemakers, ICDs and tiered therapy ICDs, these devices will becollectively referred to as “ICD/pacemakers”. It will be appreciated bythose of skill in the art that discussions herein of the pacingfunctions of an implantable device generally refer only to pacemakersand/or ICDs having pacing capabilities, whereas discussions ofcardioverting-defibrillating functions generally only refer to ICDs withor without pacing capabilities.

The pacing functions of ICD/pacemakers are described as eithersingle-chamber or dual-chamber systems. A single-chamber ICD stimulatesand senses the ventricular chamber of the heart. A dual-chamber systemstimulates and/or senses in two chambers of the heart (an atrium and aventricle). Dual-chamber systems may typically be programmed to operatein either a dual-chamber mode or a single-chamber mode.

A three-letter code (sometimes expanded to a five letter code) is usedto describe the basic mode in which the ICD/pacemaker is operating. Thethree-letter code concerns how the device operates to sense the need forand provide electrical stimulation to the heart. A fourth position (whenused) identifies the degree of programmability and rate modulation ofthe device, and a fifth position (when used) refers to electricalstimulation therapy for the primary treatment of tachycardias andfibrillations.

The first position of the three letter pacemaker code identifies thechamber to which the electrical stimulus is delivered. If the device isnot capable of bradycardia support pacing, an “O” occupies this firstposition. If the unit paces in the ventricle, this is identified by a“V”; if it paces in the atrium, the first position is identified as an“A”. If stimuli can be delivered to either the atrium or ventricle, theletter “D” is used to reflect dual-chamber stimulation.

The second position of the pacemaker code identifies the chamber orchambers in which sensing occurs. Sensing is the ability of thepacemaker to recognize the intrinsic electrical activity of the heart.The letters used in this position are identical to those used in thefirst position, i.e., “V” for ventricular sensing; “A” for atrialsensing; “D” for dual-chamber sensing; and “O” if no sensing capabilityis present.

The third position of the pacemaker code identifies the way thepacemaker responds to a sensed signal. An “I” means that the pacemakerwill be inhibited. The inhibited mode of response indicates that whenthe pacemaker senses or sees an intrinsic electrical signal, it inhibitsits own output pulse and resets one or more internal timers within thepacemaker's circuitry. The other basic response is represented by a “T”,which means triggered. The triggered mode of response indicates thatwhen the pacemaker senses an intrinsic electrical signal, it not onlyresets various internal timers within the pacemaker, it also initiatesor releases a stimulus in response to that sensed event.

The most sophisticated response mode is represented by a “D” in thethird position and refers to both modes of sensing response. Mostcommonly, a sensed signal arising from the atrium and sensed on theatrial channel of a dual-chamber pacemaker will inhibit the atrialoutput but trigger a ventricular output after a brief delay (the AVdelay). If a native ventricular depolarization does not occur before theAV delay timer completes, a ventricular stimulus will be released at theend of this AV delay. If a native ventricular signal is sensed withinthe AV delay, the ventricular output will be inhibited and other timerswill be reset. If a native ventricular signal is sensed before theatrial stimulus is released, both the atrial and ventricular outputpulses will be inhibited and the various timers will be reset.

In order to perform their pacing and/or cardioverting-defibrillatingfunctions, pacemakers and ICDs must have an energy source, e.g., abattery. Because replacement of the battery requires explantation of thedevice and implantation of a new device, it is critical that the batterylast as long as possible. A popular mode of operation for dual-chamberpacemakers, and, increasingly, ICDs having pacing capabilities, is the“DDD” mode. In this mode, the device provides electrical stimuli to bothan atrium and a ventricle of the heart, senses electrical activity inboth the atrium and ventricle and provides both inhibited and triggeredresponses to sensed electrical activity. Operating an ICD or pacemakerin the DDD mode consumes more battery energy than a mode of onlysingle-chamber sensing and/or stimulation. For example, it is estimatedthat an ICD that also provides DDD pacing consumes roughly 20% morebattery current than one that provides VVI pacing only. This translatesinto a reduction in longevity for the typical ICD of about one year.

ICD/pacemakers having DDD pacing capability are advantageous in thatthey are able to provide more appropriate therapy to the patient thansuch devices operating in, for example, VVI mode where stimulation andsensing occur only in the ventricle and the mode of response is only byinhibition of the pacing signal. Although VVI pacing may be usuallysufficient for the patient, having DDD pacing available provides morecomplete therapy to the patient. Thus, what is needed are pacemakers andICDs capable of providing DDD pacing without such a dramatic increase inenergy consumption.

SUMMARY OF INVENTION

The subject matter disclosed and claimed herein advantageously addressesthe above and other needs by providing an improved method of operationfor ICD/pacemakers, which method provides for battery conservation inDDD pacing devices. Further provided herein are improved ICD/pacemakersemploying the improved method.

Generally, the improved method provides for the deactivation of one ormore power-consuming features of the implantable medical device duringperiods when those features are not critical or are not needed. Forexample, most ICD patients infrequently require pacing support. Further,when pacing support is required, it is usually needed only for a shorttime. Thus, in a most preferred embodiment, the improved methodcontemplates deactivation of the atrial sense amplifier during normaloperation of an ICD, leaving active only the ventricular sense amplifierand pacing circuitry. Upon the occurrence of predefined events, forexample excessive sequential ventricular pacing, the atrial senseamplifier is activated, and DDD pacing, if appropriate, is initiated.The atrial sense amplifier then remains activated until one or morepredefined events occur, such as a return to a ventricular rate abovethe brady limit, at which point the atrial sense amplifier is againdeactivated.

In an alternative embodiment, an improved method of operation of an ICDis contemplated, which method includes activation of the atrial senseamplifier and initiation of DDD pacing for a predefined period of timeimmediately following the administration of cardioversion and/ordefibrillation therapy by the ICD. Since administration ofcardioversion-defibrillation therapy can give rise to a bradycardiaepisode, this improved mode of operation is advantageous.

With respect to patients requiring only pacing support, rather thancardioversion-defibrillation and pacing, the improved method may beemployed in DDD pacing pacemakers, thereby extending the useful life ofthose devices. For example, some pacemaker patients are amenable tosingle-chamber pacing support, but may benefit, from time to time, fromdual-chamber pacing support. The general method is the same, that is,the atrial sensor remains deactivated until predefined cardiac events,sensed by the ventricular sense amplifier, suggest a need fordual-chamber pacing. At this point, the atrial sense amplifier isactivated, and, if appropriate, a DDD mode of pacing is begun. Once thepatient's heart rate has returned to within predefined acceptablelimits, the atrial sense amplifier is again deactivated. As will beappreciated by those of skill in the art, an “acceptable heart rate” maybe determined in numerous ways, for example, a return to a rate abovethe patient's brady limit or recordation of a substantial number ofP-waves followed by R-waves are indicative of a stable heart rate thatno longer requires DDD pacing. Similarly, examples of events that may bemonitored for the purpose of determining when DDD pacing may benecessary include an excessive frequency of ventricular pacing and/or anexcessive number of sequential ventricular pacing events. Thus, usingthe improved method described and claimed herein, these pacemakerpatients may advantageously use their implanted device for a longerperiod of time.

Also provided herein are implantable ICDs and pacemakers employing thesenew methods to extend the useful lives thereof. In preferredembodiments, these devices include means for generating stimulationpulses; first sensing means for sensing electrical activation of theheart at the right and/or left ventricle; second sensing means forsensing electrical activation of the heart at the right and/or leftatrium; activation/deactivation means, responsive to the first and/orsecond sensing means, for activating and deactivating the second sensingmeans and/or the means for generating stimulation pulses; microprocessorcontrol and timing circuits; a clock; and a power source, such as abattery. In a most preferred embodiment, the implantable device furtherincludes memory for storing information on the operation of the deviceas well as patient-specific information and data; and telemetry meansfor receiving/sending information and data from/to the memory andfrom/to an external programmer. Additionally, if the implantable deviceis an ICD, cardioverter-defibrillator circuitry is included.

Thus, it is a feature of the present invention to provide an implantablemedical stimulating device, and method of operating such device, whichconserves the limited energy of the device's battery.

It is a another feature of the invention to provide an implantablemedical device that can operate in at least two modes, and wherein thedevice automatically operates in whichever mode consumes the leastpower, except when conditions require operation in another mode.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the presentinvention will be more apparent from the following more particulardescription thereof, presented in conjunction with the followingdrawings wherein:

FIG. 1 is a block diagram of a representative microprocessor-basedimplantable cardioverter-defibrillator that may be used with the presentinvention;

FIG. 2 is a flow chart illustrating a preferred embodiment of theimproved method described herein;

FIG. 3 is a flow chart of a preferred embodiment of the improved methoddescribed herein wherein ventricular pacing is used to determine thetime for activation of the atrial sense amplifier; and

FIG. 4 is a flow chart of an alternative preferred embodiment of theimproved method described herein wherein cardioversion-defibrillationevents are used to determine the time for activation of the atrial senseamplifier.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description is of the best mode presently contemplated forcarrying out the invention. This description is not to be taken in alimiting sense, but is made merely for the purpose of describing thegeneral principles of the invention. The scope of the invention shouldbe determined with reference to the claims. Provided herein is a methodof operation of implantable cardioverter-defibrillators (ICDs) and/orpacemakers, which method prolongs the battery life of the implantabledevice by deactivating certain power-consuming features while not neededand reactivating those features as necessary. Also provided herein areimproved devices employing these methods.

In one aspect, the method provided herein involves operating anICD/pacemaker, having DDD pacing capabilities, with the atrial senseamplifier in a deactivated state until it is determined by the devicethat DDD pacing may be necessary. For example, in a particularembodiment, the activity of the ventricular pulse generator is monitoredand the atrial sense amplifier activated when a predefined number ofsequential ventricular pacing events have occurred and/or whenventricular pacing episodes have occurred with a predefined, excessivefrequency. Once the atrial sense amplifier has been activated, bothatrial and ventricular activity are monitored to determine whether DDDpacing is actually required. If it is confirmed that DDD pacing isneeded, it is begun; if DDD pacing is not needed, then heart activity ismonitored to verify proper AV synchrony and the sense amplifier is againdeactivated. Thus, in this embodiment, the atrial sense amplifierremains inactive and the device effectively operates in a VVI mode untilthere is a specific indication that DDD pacing may be needed, therebysaving the energy normally expended by having the atrial sense amplifieractivated at all times.

In addition to the improved method described herein, which may beemployed alone or in conjunction with the embodiment just described, anembodiment is described for use in ICDs. In this embodiment, the atrialsense amplifier is not activated until either a cardioversion ordefibrillation episode is detected by the device. When this occurs, theatrial sense amplifier is activated and DDD pacing and sensing isinitiated in order to ensure the heart returns to an acceptable level offunctioning. As will be appreciated by those of skill in the art, thisparticular embodiment is most appropriate for patients that do notrequire DDD pacing at times other than immediately followingcardioversion-defibrillation therapy. For such patients, thisalternative embodiment will be tremendously beneficial, as the longevityof the implanted ICD will be dramatically improved as compared to thepresently used ICD devices employing full-time DDD pacing capabilities.

In a further preferred embodiment of the improved method describedherein, the atrial sense amplifier of an ICD, having pacingcapabilities, is activated under both of the previously describedsituations, that is, both when the frequency and/or number ofventricular pacings is excessive (i.e., greater than a predeterminedamount) and following tachycardia episodes. If the event triggeringactivation of the atrial sense amplifier is a tachycardia episode, theninitiation of DDD pacing/sensing is automatic. If, on the other hand,the triggering event is an excessive (i.e., greater than a predeterminedamount) frequency of ventricular pacings or similar indication of theneed for DDD pacing, then the atrial sense amplifier is activated.However, DDD pacing is preferably initiated only if there is no atrialactivity sensed by the atrial sense amplifier or the PR delay (the timebetween the intrinsic P-wave and the intrinsic R-wave) is longer than apredefined threshold. Although the atrial sense amplifier will beactivated more often under this most preferred method, the energysavings compared to an ICD/pacemaker operating with a full-time atrialsensor is still significant. Further, this energy savings is realizedwith little or no risk to the patient.

In another aspect, the present invention is directed to an improvedICD/pacemaker employing one or more of the improved methods. Forexample, as described in detail below, the implantable device includes acontrol circuit, which preferably is microprocessor-based, that receivesinformation from a ventricular sense amplifier and an atrial senseamplifier, that controls the issuance of stimulation pulses from aventricular pulse generator and/or atrial pulse generator andactivates/deactivates the atrial sense amplifier based upon theinformation received from the ventricular sense amplifiers and/or theoperation of the pulse generators. Additionally, where the implantabledevice has cardioverting-defibrillating capabilities, the controlcircuit may, and preferably does, control the activation/deactivation ofthe atrial sense amplifier based upon the operation of thecardioversion-defibrillation circuitry.

It will be appreciated by those of skill in the art that the presentinvention may be used with various types of implantable medical devices,including implantable cardioverter-defibrillators (ICDS) or implantabledual-chamber pacemakers or, most preferably, implantablecardioverter-defibrillators having dual-chamber pacing capabilities. Tobetter understand the invention it will first be helpful to have anunderstanding of the basic functions performed by a typical implantablemedical device with which the invention is used. To that end, referenceis first made to FIG. 1, where there is shown a simplified functionalblock diagram of an ICD/pacemaker device 10 having dual-chamber pacingcapabilities.

It is the primary function of an ICD device to sense the occurrence ofan arrhythmia and to automatically apply an appropriate electrical shocktherapy to the heart 12 in order to terminate the detected arrhythmia.Thus, the ICD/pacemaker 10 shown in FIG. 1, includes amicroprocessor-based control and timing circuit 22 (hereinafter a“control/timing” circuit) that controls the atrial (A PACE)andventricular (V PACE) pulse generating means 28 and 30, respectively, andcontrols the cardioversion-defibrillation circuitry 38 of the device.The cardioversion-defibrillation circuitry 38 generates outputelectrical stimulation pulses of moderate to high energy as necessary tocardiovert or defibrillate the patient's heart 12. Thus, for example,electrical pulses having energies of from 1 to 10 Joules (moderate) or11 to 40 Joules (high), as controlled by the control/timing circuit 22are delivered, as appropriate. Such moderate or high energy pulses arepreferably applied to the patient's heart 12 through a lead 14 whichincludes shocking electrodes 19 and 21, typically placed in the superiorvena cava (SVC) and the right ventricle. While only two shockingelectrodes are shown in FIG. 1 in association with thecardioversion-defibrillation circuitry, it is to be understood thatadditional cardioversion-defibrillation leads and electrodes may be usedas desired or needed in order to efficiently and effectively apply theshock treatment generated by the cardioversion-defibrillation circuitry38 to the patient's heart 12.

Also illustrated in FIG. 1, there are two additional electrodes (anatrial electrode 32 and a ventricular electrode 34) coupling theICD/pacemaker 10 to the heart 12. The ventricular electrode 34 ispreferably formed at the distal end of lead 14 and is in contact withone of the ventricles of the heart 12 and an atrial lead 16 having anatrial electrode 32 is in contact with one of the atria of the heart 12.Leads 14 and 16 are electrically and physically connected to theICD/pacemaker 10 through a connector 33 that forms an integral part of ahousing 35, typically conductive, wherein the circuits of theICD/pacemaker are housed. In the body of the CD/pacemaker 10, andelectrically connected to the connector, is a protection network 31which electrically protects the circuits within the ICD/pacemaker 10from excessive shocks or voltages that could appear on the electrodes32, 34 in the event such electrodes were to come in contact with a highvoltage signal, e.g., from a defibrillating shock.

The leads 14,16 carry stimulating pulses to the electrodes 34, 32 from aventricular pulse generator (V PACE) 30 and an atrial pulse generator (APACE) 28, respectively. Further, electrical signals from the atria arecarried from the electrode 32 through the lead 16 to the input terminalof an atrial sense amplifier (A SENSE) 26, and electrical signals fromthe ventricle are carried from the electrode 34 through the lead 14 tothe input terminal of a ventricular sense amplifier (V SENSE) 24. It isthe function of the sense amplifiers 26, 24 to sense the activity of theheart 12 as manifest by the presence of certain electrical signalssensed through the electrodes, 32 and 34, respectively. That is, as isknown in the art, R-waves occur upon the depolarization, and hencecontraction, of ventricular tissue; and P-waves occur upon thedepolarization, and hence contraction, of atrial tissue. Thus, bysensing R-waves and/or P-waves through the ventricular 24 and/or atrial26 sense amplifiers, respectively, and providing such sensed signals tothe control/timing circuit 22, the control/timing circuit 22 is able tomake a determination as to the rate and regularity of the patient'sheart beat. Such information, in turn, allows the control/timing circuit22 to determine whether the heart 12 of a patient is experiencing anarrhythmia.

The control/timing circuit 22 then generates trigger signals that aresent to the atrial pulse generator 28 and/or the ventricular pulsegenerator 30. These trigger signals are generated each time that astimulation pulse is to be generated by the respective pulse generator28 or 30. Similarly, where the arrhythmia detected by the control/timingcircuit 22 is a severe tachycardia or a fibrillation, the control/timingcircuit 22 generates trigger signals to the cardioversion-defibrillationcircuitry 38 as required to administer cardioversion and/ordefibrillation therapy.

The control/timing circuit 22 preferably has a memory circuit 36 coupledthereto wherein the operating parameters used by the control/timingcircuit 22 are stored. This memory circuit 36 allows the operatingparameters of the device to be programmably stored and modified asrequired in order to customize the ICD/pacemaker's operation to suit theneeds of a particular patient. Further, data sensed during the operationof the ICD/pacemaker 10 may be stored/recorded in the memory 36 forlater retrieval and analysis.

The memory 36 may take many forms and may be subdivided into manydifferent memory blocks or sections (addresses) as needed to storedesired data and control information. A feature of the present inventionis the ability to store within the memory 36 of the device informationpertaining to frequency and number of ventricular paces performed by theICD/pacemaker 10 and/or pertaining to the administration ofcardioverting and/or defibrillating therapy by the ICD/pacemaker 10.

Advantageously, in the preferred embodiments herein, the operatingparameters of the implantable ICD/pacemaker 10 may be non-invasivelyprogrammed into the memory 36 through a telemetry circuit 42 intelecommunicative contact with an external programmer 44 by way of asuitable coupling coil 43. The coupling coil 43 may serve as an antennafor establishing a radio frequency (RF) communication link 45 with theexternal programmer 44; or the coil 43 may serve as a means forinductively coupling data to/from the telemetry circuit 42 and from/tothe external programmer 44 as is known in the art. See for example U.S.Pat. Nos. 4,809,697 (Causey, III et al.) and 4,9442,299 (Silvian)incorporated herein by reference in their entirety. Further, suchtelemetry circuit 42 advantageously allows status information relatingto the operation of the ICD/pacemaker 10 as contained in thecontrol/timing circuit 22 and/or memory 36 to be sent to the externalprogrammer 44 through the established link 45.

The control/timing circuit 22 includes appropriate processing and logiccircuits for analyzing the outputs of the sense amplifiers 26 and 24 anddetermining if such signals indicate the presence of an arrhythmia.Typically, the control/timing circuit 22 is based on a microprocessor orsimilar processing circuit, which includes the ability to process ormonitor input signals (data) in a prescribed manner, e.g., as controlledby program code stored in a designated area or block of the memorycircuit 36. The details of the design and operation of thecontrol/timing circuit 22 are not critical to the present invention.Rather, any suitable control/timing circuit 22 may be used that carriesout the functions described herein. The use, design and operation ofmicroprocessor-based control circuits to perform timing and dataanalysis functions is well known in the art.

The ICD/pacemaker 10 additionally includes a battery 40, which providesoperating power to all of the circuits of the ICD/pacemaker 10 via apower signal line 39 and a clock 41 which is used for timing the variousfunctions of the device and which is in communication with thecontrol/timing circuit 22. It will be appreciated by those of skill inthe art that the clock 41, though illustrated as separate from thecontrol/timing circuit, may optionally be integral thereto.

Within the control/timing circuit 22 is an activation/deactivationcircuit 46 in communication with the atrial sense amplifier 26 via powercontrol path 47. The power control path 47 preferably causes a switch,e.g., a transistor or the like, to enable and/or disable the supply ofpower (supplied from the battery 40 via path 39) to the atrial senseamplifier 26. In a preferred embodiment of the ICD/pacemaker 10 theactivation/deactivation circuit 46 activates and/or deactivates theatrial sense amplifier 26 in response to data sensed by the ventricular24 and/or atrial 26 sense amplifiers, and/or operation of thecardioversion-defibrillation circuit 38, and/or operation of theventricular pulse generator 30.

As needed for certain applications, the ICD/pacemaker 10 may furtherinclude at least one sensor 48 that is connected to the control/timingcircuit 22 of the ICD/pacemaker 10. As illustrated in FIG. 1, thissensor 48 is optionally included within the ICD/pacemaker 10. It will beappreciated by those of skill in the art that the sensor may also beexternal to the device, either implanted elsewhere within the patient orcarried by the patient, or may be absent altogether. A type of sensorcommonly employed in ICD/pacemakers is an activity sensor, such as apiezoelectric crystal, that is mounted to the case of the pacemaker.Other types of sensors are also known, such as sensors that sense theoxygen content of blood, respiration rate, pH of blood, body motion andthe like. The presence of such a sensor is not critical to the inventionand thus, the type employed, if any, is also not critical to theinvention contemplated herein. Any sensor or combination of sensorscapable of sensing one or more a physiological or physical parametersrelatable to the rate at which the heart should be beating (i.e.,relatable to the metabolic need of the patient), and/or relatable towhether a tachyarrhythmia is likely to soon occur can be used. Suchsensors are commonly used with “rate-responsive” ICD/pacemakers in orderto adjust the rate (pacing cycle) of the device in a manner that tracksthe physiological and metabolic needs of the patient. It will beappreciated by those with skill in the art that other hardware,software, and hardware/software combinations may be included within theICD/pacemaker device without affecting the operation or usefulness ofthe present invention. Thus, any pacing device designed to pace in boththe atrium and ventricle of the heart is amenable for use in the presentinvention.

It is noted that leads 14 and 16 of FIG. 1 are illustrated as being incontact with the right ventricle and atrium, respectively, of the heart12. As is well known to those of skill in the art, these leads may be ineither the right or left atrium or the right or left ventricle. Further,as will be appreciated, the atrial lead may be positioned within thesuperior vena cava, rather than in either atria, in order to detectdepolarization in the atrium. The positioning of the leads within theheart or near the heart may be in any manner known to those with skillin the art. However, the invention described herein is designed tooperate in dual-chamber pacing devices. Therefore, at least one leadmust be positioned in contact with or in proximity to at least oneatrium, and at least one lead must be in contact with or in proximity toat one least ventricle, such that each and/or both chambers may besensed and/or paced as necessary.

Furthermore, as discussed above, the control/timing circuit 22 may berealized using a variety of techniques and/or circuits. The preferredtype of control/timing circuit 22 is a microprocessor-based controlsystem. It is noted however, that the control/timing circuit 22 may alsobe realized using a state machine. Indeed any type of control circuit orsystem could be employed for the control/timing circuit 22. The presentinvention is thus not concerned with the details of the control/timingcircuit 22; rather, it is concerned with the end result achieved by thatcircuit. That is, as long as the control/timing circuit 22 controls theoperation of the ICD/pacemaker 10 (or similar medical device) such thatthe desired functions are achieved as set forth herein (e.g., asdescribed in the flow charts of FIGS. 2-4), it matters little what typeof control/timing circuit or system is used. Those of skill in theimplantable medical device art, given the teachings presented herein,will be able to fashion numerous and varied types of control circuits orsystems capable of achieving the desired device control.

Representative of the types of control systems that may be used with theinvention is the microprocessor-based control system described in U.S.Pat. No. 4,940,052 entitled “Microprocessor Controlled Rate-ResponsivePacemaker Having Automatic Rate Response Threshold Adjustment”.Reference is also made to U.S. Pat. Nos. 4,712,555 and 4,944,298,wherein a state machine type of operation is described; and U.S. Pat.No. 4,788,980 wherein the various timing intervals used within thepacemaker and their inter-relationships are more thoroughly described.Each of the '052, '555, '298 and '980 patents are incorporated herein,in their entirety, by reference.

Turning now to the flow chart illustrated in FIG. 2, a preferred methodof operation according to the present invention is illustrated. In thisflow chart as with those illustrated in FIGS. 3 and 4, each main step ofthe method is depicted in a “box” or “block”, with each block having areference numeral assigned thereto for reference purposes. Beginningthen with block 60 of FIG. 2, the ventricular sense amplifier 24 is ONand the ICD/pacemaker 10 is operating in accordance with itspreprogrammed parameters, that is, the preprogrammedcardioversion-defibrillation and pacing routines. In block 62 a firstdetermination is made as to whether DDD pacing is required. If DDDpacing is not required (the NO branch of block 62), the system returnsto block 60 and continues to operate in its preprogrammed manner ofsingle-chamber sensing and pacing. If it is determined that DDD pacingis required (YES branch of block 62), the atrial sense amplifier 26 isactivated (block 64) via power control path 47. Once the atrial senseamplifier 26 has been activated, the need for DDD pacing is confirmed(block 66), e.g., by lack of intrinsic atrial activity. If it isconfirmed that DDD pacing is required (YES branch of block 66), then DDDpacing is initiated (block 68). After initiation of DDD pacing, it isdetermined whether the patient's heart beat is within acceptable limits(block 70). If the heart rate (HR) or PR delay are not within acceptablelimits (NO branch of block 70), the system returns to determiningwhether or not DDD pacing is still required (block 66), and if so, DDDpacing is continued. If it is determined that the heart rate (HR) and PRdelay are within acceptable limits (YES branch of block 70), then theatrial sense amplifier 26 is deactivated (block 72) via power controlpath 47. If, when the system attempted to confirm whether DDD pacing wasrequired (block 66), it was determined that no such pacing was necessary(NO branch of block 66), then the atrial rate is tracked and AVsynchrony is maintained (block 74). The next inquiry is whether or notthe heart rate (HR) or PR delay are within acceptable limits (block 76).If the heart rate is within acceptable limits (YES branch of block 66),the atrial sense amplifier 26 is deactivated (block 72). If on the otherhand, it is determined that the heart rate (HR) or PR delay are notwithin acceptable limits (NO branch of block 76), the system returns todetermining the need for DDD pacing (block 66). Once the atrial senseamplifier 26 has been deactivated (block 72) the system returns to thestarting point (block 60) with only the ventricular sense amplifier 24ON and single-chamber sensing and pacing occurring, i.e., VVI mode.

Turning now to FIG. 3, a flow chart illustrating a preferred embodiment,wherein the operation of the pacing routines provides an initialindication that atrial sense amplifier activation is needed, ispresented. This process begins in block 100 with the ventricular senseamplifier 24 ON and the ICD/pacemaker 10 operating in accordance withits preprogrammed single-chamber sensing and pacing routines. A firstdetermination is made (block 102) whether an excessive frequency ofventricular pacing episodes has occurred or an excessive number ofsequential ventricular paces has occurred. Those of skill in the artwill appreciate that defining what is an “excessive” frequency or numberof pacings may be readily determined by a physician using techniqueswell known in the pacing art and that such determinations are oftenpatient-specific. Alternatively, the ICD/pacemaker 10 may bepreprogrammed with defined excessive frequency and number valuesrepresentative of the average numbers expected to indicate a need fordual-chamber sensing and pacing. Again such values are well known to andreadily determined by those of skill in the art and will depend upon thepatient's specific type of heart ailment.

Continuing with FIG. 3, if no excessive number or frequency ofventricular pacings has occurred (YES branch of block 102), then theatrial sense amplifier 26 is activated (block 104) via power controlpath 47. If no excessive ventricular pacing has occurred (NO branch ofblock 102), and thus there is no preliminary indication that DDD pacingmay be necessary, the device returns to its normal operation with onlythe ventricular sense amplifier 24 activated. Once the atrial senseamplifier 26 is activated (block 104), the need for DDD pacing isconfirmed (block 106). If it is determined that DDD pacing is notrequired (NO branch of block 106) (e.g., because intrinsic atrialactivity is detected), then the atrial rate is tracked and AV synchronyis maintained (block 114). If, on the other hand, DDD pacing isconfirmed necessary (YES branch of block 106), DDD pacing is initiated(block 108). Once the patient's heart rate (HR) and PR delay havereturned to within acceptable limits (YES branch of block 110), theatrial sense amplifier 26 is deactivated (block 112) via power controlpath 27 and the program returns to the starting position (block 100)with only the ventricular sense amplifier 24 activated. So long as theheart rate (HR) and the PR delay of the patient is not within acceptablelimits (NO branch of block 110), the ICD/pacemaker 10 continues toconfirm the need for DDD pacing (block 106).

FIG. 4 illustrates a preferred embodiment of the present inventionwherein activation of the atrial sense amplifier 26 is triggered by thedetection of ventricular arrhythmia. This process begins in block 150with the ventricular sense amplifier 24 ON and the ICD/pacemaker 10operating according to its preprogrammed single-chamber sensing andpacing routines. An initial determination is made as to whether eithercardioversion or defibrillation episode has been detected (block 152).If no cardioversion or defibrillation episode has been detected by theICD/pacemaker 10 (NO branch of block 152), the ICD/pacemaker 10 returnsto the starting point (block 150). If a cardioversion and/ordefibrillation episode has been detected by the ICD/pacemaker 10 (YESbranch of block 152), the atrial sense amplifier 26 is activated (block154) and DDD pacing and sensing is, preferably, automatically initiated(block 156). Next, it is determined whether the heart rate (HR) and PRdelay are within acceptable limits (block 158). If the heart rate (HR)and PR delay are not within acceptable limits (NO branch of block 158),DDD pacing/sensing is continued (block 156). If it is determined thatthe patient's heart rate (HR) and PR delay are within acceptable limits(YES branch of block 158), the atrial sense amplifier 26 is deactivated(block 160) via power control path 27 and the program returns to itsstarting point (block 150) with only the ventricular sense amplifier 24ON and the preprogrammed routines operating.

While the invention herein disclosed has been described by means ofspecific embodiments and applications thereof, numerous modificationsand variations may be made thereto by those skilled in the art withoutdeparting from the scope of the invention which is set forth in theclaims. For example, while power control of the atrial sense amplifierhas been expressly shown associated with the current ICD/pacemaker modeof operation, one of ordinary skill would also recognize that powercontrol could also be extended to control other portions of thecircuitry of the ICD/pacemaker, e.g., the atrial and/or ventricle pulsegenerators, that are not needed in a particular operating mode.

What is claimed is:
 1. In an implantable stimulation device having an atrial sense amplifier, a method of reducing power consumption comprising: activating the atrial sense amplifier in response to an occurrence of a predetermined number of sequential ventricular pacing events; monitoring heart activity; and deactivating the atrial sense amplifier to reduce power consumption when the heart activity is within acceptable limits.
 2. The method of claim 1, further comprising confirming a need for atrial sensing subsequent to activating the atrial sense amplifier and initiating an atrial sensing mode.
 3. The method of claim 1, wherein: the monitoring step comprises detecting a parameter related to ventricular activity; and the deactivating step comprises detecting when the parameter related to ventricular activity is within the acceptable limits.
 4. The method of claim 3, wherein the detecting when a parameter related to ventricular activity is within acceptable limits comprises at least one of: detecting that a ventricular rate has returned to within predefined acceptable limits; detecting that the ventricular rate is above a predetermined base rate; detecting a P-R interval within the acceptable limits; and detecting that a substantial number of P-waves are followed by respective R-waves which is indicative of a stable ventricular rate that no longer requires atrial sensing. 